This invention relates generally to ion implantation machines used to process integrated circuit wafers and more particularly to methods and apparatus used to measure particulate contamination of wafers processed in such machines.
Ion implantation machines can be used to selectively implant impurities into semiconductor wafers to vary their electrical properties. This is accomplished by forming an energetic beam of ions of a desired impurity, such as boron or phosphorus, and directing the beam to the wafer surface. Unfortunately, the ion beam can transport particulates from the beam line to the wafer surface thereby causing particulate contamination of the wafer. With the continuing trend towards larger integrated circuits and smaller chip geometries, even small (e.g. submicron) sized particles can shadow a surface of the wafer and prevent ion implantation beneath the particle. This can, of course, adversely effect the ultimate yield of the implanted wafer. Therefore, the monitoring of particulate generation caused by ion implantation is of critical concern to semiconductor process engineers.
The number of "killer particles" on a wafer can be determined by inspection of the wafer after it has been processed within the ion implantation machine. However, it is far more desirable to obtain an indication of the particle level within the ion implantation machine while the wafers are still being processed so that trouble points during the process can be isolated and so that possible corrective actions can be taken.
High Yield Technology, Inc. of Sunnyvale, Calif. markets an in-situ particle flux monitor as model PM-150 which can be mounted in the load-lock, exhaust line, or vacuum chamber of an ion implantation machine. In a paper entitled "Real-Time, In-Situ Particle Monitoring in a High Current Ion Implantation Production Bay" by Weisenberger et al., Proceeding of the Seventh International Conference on Ion Implantation Technology, Kyoto, Japan, June 7, 1988, the use of a High Yield Technology PM-150 monitor within the end station of a Varian ion implantation machine is described. While some positive results were reported, the actual number of particles detected by the PM-150 within the Varian ion implantation machine was low due to the low concentration of particulates at the sensing aperture of the monitor and due to the relatively low sensitivity and response time of the PM-150 monitor.
A partial solution to the above problem might be to substitute a higher speed, more sensitive monitor for the PM-150. However, such monitors are not readily commercially available and, if they were, would suffer a sensitivity to x-rays generated by the impingement of the ion beam on surfaces within the vacuum chamber of the ion implantation machine. In consequence, the prior art does not disclose a sensitive, reliable particle detector which can be placed within the vacuum chamber of an ion implantation machine.